1. Field Of The Invention
The present invention relates to tracking error signal generating devices and more particularly to a tracking error signal generating device for recording on or reproducing from an optical recording medium.
2. Background Art
A system for recording or playing back data on an optical recording disk is equipped with a so-called tracking error signal generating device for indicating how far a data detecting light spot of a pickup is separated from a recording track in the direction perpendicular to the recording track involved (radial direction of the disk). FIG. 1 shows an example of such an operation.
In FIG. 1, there is shown a four-segment light receiving element 1. The data detecting spot light of a pickup is incident upon a recording surface on the light receiving surface of the four-segment light receiving element 1. The four-segment light receiving element 1 is divided by two intersecting straight lines .alpha. and .beta. into four elements 1a-1d adjacent to each other so as to form four light receiving areas, as shown in FIG. 1. The outputs of the elements 1a and 1c disposed symmetrically with respect to the straight lines .alpha. and .beta. are respectively supplied via buffer amplifiers 102 and 103 to an adder 104, whereas the outputs of the elements 1b and 1d disposed at 90.degree. thereto are respectively supplied through buffer amplifiers 105 and 106 to another adder 107.
The outputs of the adders 104 and 107 are subjected to waveform rectification by waveform rectifier circuits 108 and 109 before being supplied to a phase difference direction circuit 110. In the phase difference detection circuit 110, the output p of the first waveform rectifier circuit 108 is supplied to the clock input terminal of a D-type flip-flop 112 and the reset input terminal of a D-type flip-flop 114. Simultaneously, the output p of the first waveform rectifier circuit 108 is supplied to an inverter 115. An inverted signal -p of the output p is sent out from the inverter 115 and supplied to the clock input terminal of a D-type flip-flop 111 and to the reset input terminal of a D-type flip-flop 113.
The output r of the second waveform rectifier circuit 109 is supplied to the reset input terminal of the D-type flip-flop 111 and to the clock input terminal of the D-type flip-flop 113. At the same time, the output r of the second waveform rectifier circuit 109 is supplied to an inverter 116. An inverted signal -r of the output r is sent out from the inverter 116 and supplied to the reset input terminal of the D-type flip-flop 112 and to the clock input terminal of the D-type flip-flop 114.
A power supply voltage is applied to the D input terminals and to the set input terminal of all of the D-type flip-flops 111 to 114. The Q outputs q.sub.1 and q.sub.2 of the D-type flip-flops 111 and 112 are supplied through an OR-gate 117 to the inverting input terminal of a differential amplifier 118, whereas the Q outputs q.sub.3 and q.sub.4 of the D-type flip-flops 113 and 114 are supplied through an OR-gate 119 to the non-inverting terminal of the differential amplifier 118. The output of the differential amplifier 118 is sent out through a LPF (low-pass-filter) 120 as a tracking error signal e.
With the thus arranged previously described four-segment light receiving element 1, it is assumed that the center of the light spot formed by the light from the recording surface on the light receiving surface coincides with the intersection of the straight lines .alpha. and .beta. and that the straight line .alpha. is in parallel with the direction in which the pattern of the intensity distribution of light from the recording surface moves as the data detection light spot follows the recording track and is reflected by the series of pits forming the track. While the data detection light spot is following the recording track, the pattern of the intensity distribution of the light within the light spot formed on the light receiving surface of the light receiving element 1 quickly moves from the element 1a to the element 1b (downward in FIG. 1). for instance, and the pattern of the intensity distribution of the light is symmetrical with respect to the straight line .alpha.. Then the phase difference between the waveform rectifying outputs p and r becomes "0" and the D-type flip-flops 111 to 114 remain in the reset state so that the outputs q.sub.1 -q.sub.4 are kept at a low level. Consequently, there is produced no level difference between the inverting input x and the non-inverting input y of the differential amplifier 118. Accordingly, the output of the differential amplifier 118 is set at the ground level, whereby the level of the tracking error signal e produced by the LPF 120 becomes equal to the ground level.
Referring to FIG. 4, the following description will be made in a case where the data detection light spot is displaced inwardly in the radial direction and the pattern of the intensity distribution of the light within the light spot formed on the light receiving surface of the four-segment light receiving element 1 is displaced away from the element 1d and toward the element 1a (to the left in FIG. 2). This displacement along the line .beta. is semistatic as the light spot more quickly moves parallel to the line .alpha.. FIG. 2(A) is a waveform chart of the waveform rectifying output p; FIG. 2(B) is a waveform chart of the inverted output -p; FIG. 2(C) is a waveform chart of the waveform rectifying output r: FIG. 2(D) is a waveform chart of the inverted output -r; FIG. 2(E) is a waveform chart of the Q output q.sub.1 : FIG. 2(F) is a waveform chart of the Q output q.sub.2 ; FIG. 2(G) is a waveform chart of the Q output q.sub.3 : FIG. 2(H) is a waveform chart of the Q output q.sub.4 ; FIG. 2(I) is a waveform chart of the out-of-phase (inverting) input x of the differential amplifier 118; and FIG. 2(J) is a waveform chart of the in-phase (non-inverting) input y of the differential amplifier 118.
When the pattern of the intensity distribution of the light is displaced away from the element 1d and towards the element 1a the waveform rectifying output p and the inverted output -p precede in phase the waveform rectifying output r and the inverted output -r by an angle corresponding to the displacement of the data detection light spot. Then the flip-flops 111 and 112 remain in the set state for a time interval corresponding to that angle so that the outputs q.sub.1 and q.sub.2 of Q are set at a high level for a time corresponding to that angle. At the same time, the flip-flops 113 and 114 are kept in the reset state and the outputs q.sub.3 and q.sub.4 of Q are left at a low level. Consequently, of the inverted input x and the non-inverted input y of the differential amplifier 118, only the inverted input x is kept at the high level for a time interval corresponding to the displacement of the data detection light spot. A negative pulse having a pulse width corresponding to the displacement of the data detection light spot along the line .beta. is produced by the differential amplifier 118 and sets the tracking error signal e produced by the LPF 120 to the negative level and further sets its absolute value proportional to the displacement of the data detection light spot.
FIG. 3 shows a time chart in a case where the data detection light spot is displaced in the opposite direction, that is, outwardly in the radial direction and the pattern of the intensity distribution of the light within the light spot formed on the light receiving surface of the four-segment light receiving element 1 is displaced away from the element 1a and toward the element 1d (to the right in FIG. 3). FIGS. 3(A) through (J) show similar signal waveforms to those shown in FIGS. 2(A) through (J). respectively.
When the pattern of the intensity distribution of the light is displaced away from the element 1a and toward the element 1d, the phases of the waveform rectifying output p and the inverted output -p are respectively delayed by an angle corresponding to the data detection light spot compared with the phases of the waveform rectifying output r and the inverted output -r. The flip-flops 113 and 114 are kept in the set state for a time corresponding to the angle involved and the outputs q.sub.3 and q.sub.4 of Q are kept at a high level for a time corresponding to the angle involved. Moreover, the D-type flip-flops 111 and 112 are held in the reset state, whereas the outputs q.sub.1 and q.sub.2 are held at the low level. Of the inverted input x and the non-inverted input y of the differential amplifier 118, only the non-inverted input y is set at a high level for a time corresponding to the displacement of the data detection light spot. Accordingly, a positive pulse having a pulse width corresponding to the displacement of the data detection light spot along the line .beta. is produced by the differential amplifier 118, thus causing the level of the tracking error signal e produced by the LPF 120 to be positive with its absolute value corresponding to the displacement of the data detection light spot.
In the conventional tracking error signal generating device, the position of the light spot formed on the light receiving surface of the four-segment light receiving element 1 moves from the position shown by a continuous line u in FIG. 4 to what is shown by a broken line v or w only when the objective lens of the pickup device containing the four-segment light receiving element 1 is caused to be displaced to effect the tracking control The deviation of the position of the data detection light spot on the recording surface of the recording medium from the recording track allows the quantities of light incident on the elements 1a through 1d to be unbalanced. However a similar case arises where the pattern of the intensity distribution of the light within the light spot formed on the light receiving surface of the four-segment light receiving element 1 becomes asymmetrical with respect to the straight line .alpha.. That is, the separate elements 1a-1d measure area integrated intensities. If the beam is asymmetrical the detected center of the beam is displaced from the geometric center. When the light spot formed on the light receiving element 1 exists in the position shown by the broken line v of FIG. 4 for instance, the outputs of the elements 1a through 1d becomes as shown in traces (A) to (D) respectively of FIG. 5 and the amplitudes of the outputs of the elements 1a through 1d are unbalanced. Consequently the phase difference between the sum components of the diagonally disposed elements which are produced by the adder circuits 104 and 107 fail to accurately correspond to the distance between the geometric center of the data detection light spot and the recording track, thus causing the tracking error signal to be offset.
The occurrence of the offsetting causes the following disadvantage. A tracking error signal shown by a continuous line u' of FIG. 6(B) is produced when the light spot formed on the light receiving surface of the four-segment light receiving element 1 exists in the position shown by a continuous line u of FIG. 4. Then, the sum of the outputs of the elements 1a through 1d changes as shown in FIG. 6(A) owing to the track jumping movement of the data detection point On the other hand, when the light spot formed on the light receiving surface of the four-segment light receiving element 1 moves to one of the positions shown by the broken lines v and w of FIG. 4, the level of the tracking error signal changes as shown respectively by broken lines v' and w' of FIG. 6(B).